(a) Field of the Invention
The present invention relates to a method of fabricating liquid crystal display, and more particularly, to a mask for forming polysilicon and a method for fabricating thin film transistor using the same.
(b) Description of the Related Art
Generally, a liquid crystal display has two substrates with electrodes, and a liquid crystal layer interposed between the two substrates. Each of the two substrates is sealed by a sealer while being spaced apart from each other by spacers. A voltage is applied to the electrodes so that the liquid crystal molecules in the liquid crystal layer are re-oriented to thereby control an amount of light transmission through the liquid crystal layer. Thin film transistors are provided at one of the substrates to control the signals transmitted to the electrodes.
Amorphous silicon is typically used to form a semiconductor layer in the thin film transistors. Generally, current mobility of the amorphous silicon-based thin film transistor is low, at about 0.5-1 cm2/Vsec. This is inadequate for directly forming a driving circuit on a substrate. Thus, a polysilicon-based thin film transistor, which has a relatively high current mobility of about 20-150 cm2/Vsec, has been developed to directly fabricate a driving circuit on the substrate.
Various methods for forming a polysilicon thin film have been proposed, including: directly depositing polysilicon layer onto a substrate at a relatively high temperature; depositing amorphous silicon layer onto a substrate and crystallizing the amorphous silicon layer at a temperature of about 600° C.; and depositing amorphous silicon onto a substrate and heat-treating the amorphous silicon layer using laser. However, as the polysilicon layer formed by using such high temperature has non-uniform crystalline particles, which deteriorate electrical characteristics of the thin film transistors, these methods are generally not applicable for substrates of liquid crystal display panels.
A sequential lateral crystallization process where the distribution of crystalline particles can be controlled in an artificial manner has been developed. In the sequential lateral crystallization process, polysilicon grains are grown perpendicular to the interface between a laser-illuminated liquid phase region and a non-illuminated solid phase region. The laser beam passes through a mask having a slit-shaped transparent portion, then, a slit-shaped liquid phase region is formed at an amorphous silicon layer. Thereafter, the liquid phase amorphous silicon is crystallized while being cooled. The growth of crystalline begins from the boundary of the solid phase region, and stops at the center of the liquid phase region. Such a process is repeated with moving the mask in the growing direction of the polysilicon grains so that the sequential lateral crystallization is performed throughout the entire target area.
In the case wherein the sequential lateral crystallization is performed while moving the mask only in the growing direction of the polysilicon grains, the size of the polysilicon grains in the moving direction of about several micrometers can be obtained. However, a size of the polysilicon grains in the perpendicular direction of the moving direction is about several thousand angstroms. Furthermore, the sizes of the polysilicon grains in a peripheral region of the mask and an edge of the slit are non-uniform. Consequently, polysilicon thin film transistors exhibit anisotropic characteristics. For example, current mobility of the thin film transistors is largely differentiated in two directions.
Thus, it is desirable to provide a method of fabricating a polysilicon layer and thin film transistors having isotropic current mobility.